1. Field of the Invention
This invention relates to fasteners such as nuts, bolts, and the like, and more particularly to a hardened fastener that is specially formed to take advantage of limitations in present-day machining processes to realize optimum hardness in the fastener.
2. Description of the Related Art
Metal alloys formed from cobalt, and/or nickel, chromium, and molybdenum such as those known in the art as MP35N, MP159, and such alloys made in conformance with the AMS 5844, and AMS 5842 standards are known for their hardness once they have been cold reduced. Similar material known as Custom Age 625 PLUS.RTM., which is a nickel-based alloy, are also useful. Cold forging, or cold working, is a process by which metals, alloys, and the like are mechanically treated as by rolling, swaging, or drawing so that the overall cross-sectional area is reduced. Generally, the cold reduction converts face-centered cubic (FCC) crystals present in the alloy to hexagonal close-packed (HCP) crystals. This provides increased mechanical strength, although it can also lead to brittleness and result in a decrease in structural integrity under stress.
There is a class of materials based upon cobalt, nickel, chromium, and molybdenum that experience high strength upon cold-working. These materials also maintain their high strength under hot conditions. These materials include those listed above, namely, MP35N, MP159, Custom Age 625 PLUS.RTM. and alloys formed under the AMS 5844, and AMS 5842 standards.
When annealed, these alloys may become soft. However, by cold working, their strength can be increased to obtain tensile strengths over 250,000 psi. Additional heat treating can also further enhance the strength up to as high as 280,000 psi.
Such hardnesses become increasingly difficult for machine dies, tools, and the like to produce further cold work. It can be seen, therefore, that the art is advanced by providing hardened fasteners whose heads are not softened by hot forging, yet can be worked so that the greatest hardness available through cold working is realized through the fastener. It would be to some advantage to realize such a fastener, and the process by which it might be achieved. By providing such a fastener, parts or components requiring fasteners with a high degree of hardness can be attached to one another and not suffer disassembly or failure during stressful or thermally energetic circumstances.